A sampler in an analog-to-digital converter (hereinafter, referred to as AD converter or ADC) generally includes a switch and a capacitor in order to sample and hold input analog signals. The switch is turned on and off at predetermined intervals. The capacitor is charged while the switch is being turned on. In the case where the switch of the sampler is formed of a semiconductor device (switching device), charge injection due to a parasitic capacitance of the device becomes a problem. Specifically, in addition to charges of the capacitor of the sampler, charges of the parasitic capacitance of the switching device also appear in an output. This becomes a cause of variations in output common-mode voltage of the sampler.
As methods of preventing the charge injection due to the parasitic capacitance, a method using a dummy switch and a method using a detector are known. In the method using a dummy switch, the dummy switch is provided parallel to the switch of the sampler to absorb the charges of the parasitic capacitance. In the method using a detector, the variations in output common-mode voltage of the sampler are detected and corrected.